Process for bonding a contact layer of silver-metal oxide material and metal contact base, and suitable contact layer

ABSTRACT

A contact facing is normally bonded to the carrier by brazing or welding, for which purpose, the bonding side has to have a suitable layer. According to the invention, prior to the bonding operation, the contact facings are treated without fusion in such a way that the metal oxide is at least partially reduced to metal at least on the solder side at the surface and in the subsurface region of the contact facing. The reduction can be carried out by heat treatment in reducing atmosphere. In the case of contact materials based on silver/iron oxide (AgFe 2  O 3  /Fe 3  O 4 ), in particular, the iron oxide may also be reduced to iron on the switching side since iron oxide is again formed after switching. Alternatively, contact facings may be provided with carbon as reducing agent on the solder side. In particular, the reduction may take place locally in this case.

BACKGROUND OF THE INVENTION

The invention relates to a method of bonding contact facings composed ofcadmium-free silver/metal oxide materials which contain at least tinoxide or iron oxide as the essential metal oxide to a metallic contactcarrier, in particular by brazing or welding. In addition, the inventionalso relates to associated contact facings.

Silver/metal oxide contact facings can be formed as individual contactpieces or as linearly extended contact profiled sections or contactstrips. Such contact facings have to be electrically and mechanicallybonded to metallic contact carriers. Contact facings of this type cannotalways be reliably bonded to the contact carriers by the known methodsof brazing or welding. Frequently, contact facings of this type aretherefore provided with a layer of pure silver. This layer can beapplied by various production methods: in the case of profiled sections,it is done by cladding the contact-material strips or by directtwo-layer extrusion, but when manufacturing molded articles it is doneby by pressing two separately introduced layers. In all cases, theapplication of the pure-silver layer is associated with an increasedtechnical complexity in the manufacture. This entails an additionalfinancial cost which results in an appreciable increase in price of suchcontact facings.

In the case of single-layer contact facings which are to be applied bysoldering to metallic contact carriers, German Offenlegungsschrift 22 60559 has already proposed dissolving the metal oxide component outchemically in order to achieve better soldering properties. The latterhas not, however, gained acceptance in practice since only the oxidessituated directly at the surface are dissolved out by the chemicalremoval of the metal oxide component. In the soldering process, however,silver is generally dissolved by the solder, with the result that aninadequate oxide removal results in a poor wettability and not,consequently, in a reliable soldering. From SU-A-1108522 it is known toreduce the solder side of contact pieces composed of silver/metal oxidematerials by means of electrochemical processes in order to make bondingto the contact carrier possible without a separate silver layer.Finally, with JP-A-5-182558 is described the manufacture of silver-metaloxide/metal compound arrangements based on silver/cadmium oxide, stripsof silver/cadmium oxide material which can contain further metal oxidesas minor constituents being heated by current heating in a N₂ /COatmosphere and a selective reduction being carried out at the surface ofthe strip. With these means a silver/cadmium solder is formed in thesubsurface region of the contact facing, which means the switchingproperties on the switching side get lost. Therefore, after the bondingof the contact pieces to the contact carrier, the contact side mustagain be oxidized to higher valency.

SUMMARY OF THE INVENTION

In contrast, the object of the invention is to render contact facingscomposed of materials based on silver/tin oxide or silver/iron oxidesolderable without separately applying a pure-silver layer.

According to the invention, the object is achieved in that, prior to thebonding operation, the contact facings are treated without fusion insuch a way that the metal oxide is at least partially reduced to metalat least on the solder side both at the surface and also in thesubsurface region of the contact facing. If the reduction parameters arechosen so that the reduced layer is sufficiently thick, a reliablesoldering can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a metallographic polished section of the surface andof the subsurface region of a contact piece composed of AgFe₂ O₃.

FIG. 2 illustrates a metallographic polished section of the surface andof the subsurface region of a contact piece composed of AgFe₂ O₃ ZrO₂.

The method according to the invention is advantageously applied in thecase of contact facings composed of AgSnO₂ Bi₂ O₃ CuO materials, on theone hand, or composed of Ag/Fe₂ O₃ or Ag/Fe₂ O₃ /ZrO₂ materials, on theother hand. In the exceptional case, a partial reduction of a metaloxide may already be sufficient.

Within the scope of the invention, the metal oxide at the surface and inthe subsurface region can be reduced in various ways. Preferably, it isdone by a heat treatment of the contact facings in reducing gasatmosphere. For this purpose, the heat treatment can be carried out in aroasting furnace or by inductive heating of the metallic contact facing.It can, however, also be done by radiation from a suitable high-energylamp, fusion of the material not yet occurring in the case of saidradiation. Suitable for the reducing atmosphere are, in particular,hydrogen, forming gas or, alternatively, carbon monoxide.

EP-A-0 288 585 proposes a method of producing a solderable or weldableunderside on silver/MeO contacts, in which the underside of the contactplatelet is to be punctiformly fused and reduced by means of a laserbeam which travels in raster fashion. For this purpose, the surface ofthe contact platelets has first to be subjected to a pretreatment inorder to prevent a reflection of the laser beam. During the coupling-inof laser radiation, local overheatings always occur and this can resultin undesirable microstructure changes.

To reduce the metal oxide at the surface and in the subsurface regionwithout fusion, it may be advantageous to provide the contact facings onthe solder side with carbon and to subject them, in this way, to a heattreatment. It is thereby possible to achieve a local reduction only inthe solder region. For this purpose, the carbon may, for example, besprayed as graphite onto the solder side.

In addition to the above methods in which the solder side isspecifically treated and the contact side of the contact facing isotherwise left unaffected if possible, the metal oxide may also bereduced to metal on all sides at the surface and in the subsurfaceregion, especially if the metal which the metal oxide comprises isinsoluble or only slightly soluble in silver. The latter method isadvantageously possible, in particular, if the contact material-containsespecially iron oxide in addition to silver. In this case, the ironoxide is also reduced to iron on the switching side, especially as aresult of roasting and the contact layer consequently contains iron inthe fresh state. Since this layer is normally only a few μm thick,however, the switching properties are not impaired. On the contrary, itappears that, even during or after the first switching of the contacts,iron oxide from the parent material also has an effect in the switchmicrostructure or that the iron is reoxidized.

Further details and advantages of the invention emerge from thedescription of exemplary embodiments below, reference being made in somecases to the micrographs. In each case in accordance with the treatmentaccording to the invention.

EXAMPLE 1

Contact facings are produced from a material having the constitutionAgSnO₂ Bi₂ O₃ CuO. Contact materials of this type and the associatedpowder-metallurgy production are disclosed in EP-A-0 164 664 and EP-A-0170 812, the use of internally oxidized alloy powders being specifiedtherein. In addition, suitable boundary conditions are adhered to forthe concentrations of the individual components of the alloy. Thecontact pieces are produced therein by sintering as a moulding.

After production of the moulded contact pieces by sintering, asubsurface reduction of the metal oxides is advantageously carried outby inductive heating in forming gas which can be specifically controlledas a function of the frequency.

Optionally, an energy radiation treatment by means of a high-energy lampis also possible for heating purposes. A high-energy lamp has an energydensity which is such that fusion and, consequently, a liquid phase doesnot yet generally exist, but a temperature distribution sufficient forthe rapid reduction of the interfering oxides is nevertheless present inthe surface region. For this heating it may be beneficial to use carbonas reducing agent instead of the reducing gas atmosphere. In particular,the solder side of contact facings may be sprayed with graphite so thatthe metal oxide is reduced only in this region.

EXAMPLE 2

Recently, silver/iron oxide materials have proved satisfactory inpractice. For example, a contact material Ag/Fe₂ O₃ 10 has beeninvestigated. Since the internal oxidation of an alloy powder isimpossible because of the low solubility of the iron in the silver,separate powders composed of silver and iron oxide were mixed togetherto produce this material. Contact facings are produced by knownsintering methods as a moulding or strip.

If contact facings of this type are roasted for half an hour at 500° C.in hydrogen, a substantially complete reduction of the iron oxide toiron results in a region of up to approximately 20 μm. In FIG. 1, theparent material of the contact piece having a composite microstructureis denoted by 1, the surface of the contact piece by 2 and the surfaceregion by 3. The region of the parent material 1 comprises a silvermatrix 5 having iron oxide particles 6 which are incorporated thereinand which form dark particles. It can be seen that no iron oxideparticles 6 are present at the surface 2 and 3 in the region 3 butbrighter iron particles which have been produced by the reduction.

A proportion of less than 10% by mass of iron in the silver after thereduction results in a reliable solder joint if a suitable solder isused. The same bonding technologies as in the prior art with a separatesilver layer are therefore possible in a known manner, provided thereduction parameters are appropriately chosen.

Since, in particular, the switching area is also reduced in theprocedure described, iron in elemental form is initially present in thefresh state in contact pieces. As a result, the temperaturecharacteristics may be slightly altered during the first switchingoperations and may adjust to the desired value only in the course of theswitching operations.

EXAMPLE 3

For application in various low-voltage contactors, materials based onsilver/iron oxide with an additional zirconium oxide component, inparticular a contact material Ag/Fe₂ O₃ 5.4/ZrO₂ 1, have provedsatisfactory. Such materials are described in the earlier,non-prior-published Patent Application WO-A-92/22080. Since silver, ironand zirconium have only a low solubility, this material is produced bypowder metallurgy by mixing the individual metal oxide powders. A stripor profiled section of this material from which separate contact piecescan be cut to length later can thereby be produced by an extrusiontechnique.

If such a strip is roasted in a roasting furnace in hydrogen at atemperature of 500° C. for a time of about half an hour, the iron oxideat the surface and in the subsurface region of the strip is reduced to adepth of approximately 200 μm. The zirconium oxide, which is present ina comparatively small amount, virtually does not yet undergo anyreaction at a heating temperature of 600° C. so that no reduction takesplace in this respect.

Cutting to length consequently yields contact pieces in which the solderside only has zirconium oxide as metal oxide. An associated micrographis shown in FIG. 2. According to FIG. 2, an internal region 11containing parent material, a surface 12 of the contact piece and asubsurface region 13 are present. It can be seen that the parentmaterial 11 comprises a silver matrix 15 containing metal oxideparticles 16 and 17 composed of Fe₂ O₃, on the one hand, and of ZrO₂, onthe other hand, the iron oxide being comparatively finely divided andthe zirconium oxide comparatively coarsely divided. In the subsurfaceregion 13, on the other hand, iron particles reduced from Fe₂ O₃ arepresent as brighter particles, but the ZrO₂ is left unaltered as coarseparticles.

It has been found that a contact piece treated in this way andcontaining silver/iron as the majority component at the surface can bereliably soldered. It is found especially in this system that theswitching properties, in particular the temperature behaviour, are notsubstantially changed since zirconium oxide is also present at theswitching side in the fresh state of the contact pieces and, inaddition, even after a few switching operations, the iron particles areoxidized as a result of energy conversion in the case of air-brakecontactors and, consequently, the specified constitution andconcentration of the contact material is regained.

What is claimed is:
 1. A method for bonding a contact layer composed ofa cadmium-free silver/metal oxide material which contains at least tinoxide or iron oxide as an essential metal oxide to a metallic contactbase, comprising the steps of:subjecting the contact layer to a heattreatment under a reducing atmosphere and without fusing taking place,reducing the metal oxide at least partially to metal on a bonding sideof the contact layer without an alloy formation, and in a subsurfaceregion of a switching side of the contact layer; and subsequentlybonding the contact layer to a metallic contact base.
 2. The methodaccording to claim 1, wherein the heat treatment is carried out in anannealing furnace.
 3. The method according to claim 2, wherein the heattreatment is carried out by inductive heating.
 4. The method accordingto claim 2, wherein the heat treatment is carried out locally byradiation with high energy density.
 5. The method according to claim 2,wherein the reducing atmosphere is hydrogen (H₂).
 6. The methodaccording to claim 2, wherein the reducing atmosphere is forming gas. 7.The method according to claim 2, wherein the reducing atmosphere iscarbon monoxide.
 8. The method according to claim 1, wherein the contactlayer is provided with carbon on the bonding side and is subjected to aheat treatment.
 9. The method according to claim 8, wherein the carbonis applied as graphite onto the bonding side.
 10. The method accordingto claim 1, wherein the contact layer is composed of AgSnO₂ Bi₂ O₃ CuOproduced by inner oxidation of alloy powder.
 11. The method according toclaim 1, wherein the contact layer is composed of Ag/Fe₂ O₃ produced bya powder mixture.
 12. The method according to claim 1, wherein thecontact layer is composed of Ag/Fe₂ O₃ /ZrO₂ produced by a powdermixture.
 13. A contact layer produced by the following process: heattreating a layer comprising silver and iron oxide (Fe₂ O₃ /Fe₃ O₄) undera reducing atmosphere without fusion; and reducing the iron oxide atleast partially to iron on a bonding side of the layer, without alloyformation, and in a subsurface region of a switching side of the layer,and on the switching side of the layer.
 14. The contact layer accordingto claim 13, further comprising zirconium oxide (ZrO₂).